Could be alot of things ... timing comes to mind as well as valve clearances. Something in the bottom end could be broken or worn out to where it's causing something to get hung up, possible connecting rod, transmission gears ... you name it. Best thing i could say cause obviously i can't see or hear it run ..... get a book ... start reading ... get some basic tools .... try to narrow it down to where exactly the noise is coming from via a stethoscope or a very long screwdriver with a solid handle ( old school stethoscope ) and see what parts of the engine are in that side ( again check the book first before taking things apart ). And just be sure to check that those parts are still available either through a dealer or second hand, never hurts to get every piece of information before hand .... I'm sure some of the guys on here can help out in some way